Project

Solar Electric Sails in Space

The idea of leaving the engine of a craft behind and using the endless fuel supply of solar or star light seems like a great way of reducing the mass of a craft. The obvious disadvantage is the dependence on a high flux of photons to give the craft the needed acceleration. For interstellar travel, light-sail craft have to depend on extremely large-scale constructions such as huge solar-power relays around Mercury and enormous Fresnel zones in the outer Solar System. Since light applies pressure to surfaces, the stream of photons can be used for propulsion in a near-frictionless environment. This concept is the background for light (or solar) sails. It is a method of space travel that negates the need for onboard fuel. Sails using the solar wind or only the light from stars are less efficient at larger distance from the Sun.

In science fiction, solar sails are encountered from the 1920s, with early models being giant, multiple-sail craft. In other works, light sails are used to propel ramjets up to ram speeds. To increase efficiency, ground based lasers can be used to push the craft – using monochromatic light increases the reflectivity of the sail material and gives more acceleration. The efficiency decreases with distance, but much slower than if the craft was riding sunlight alone. The laser beam can be refocused by gigantic Fresnel zones. Light sails will have enormous areas, but the craft will carry no fuel or bulky engines. Alternatives to light sails include microwave sails, particle sails, magnetic sails, and laser or solar thermal or electrical propulsion.

Basically, the light sail is a use of James C. Maxwell’s discovery in 1873 that light reflected in a mirror applied pressure to the mirror. Since photons according to Einstein have mass then, by using the rather low friction coefficient of space, a craft is able to travel from A to B without having to carry bulky propulsion devices and especially without the need for onboard fuel. This is a large plus in terms of logistics. The fuel is supplied from nearby stars or by high-power lasers.

Arthur C. Clarke wrote “Sunjammer” in the early 1960s, but one of the earliest sources of light sails is a compilation of works by Cordwainer Smith, the name under which Dr. Paul M. A. Linebarger wrote science fiction in the 1950s. In this story the first manned interstellar ships are propelled by light sails. The smallest, earliest sails were only about 5000 km2. They were made of “tissue metal” – probably a fine mesh to give lightness to these enormous sails. Each ship carried many sails, and was steered by the manipulation of the sails much like sail-carrying ships today. Larry Niven is a later science-fiction writer who uses light sails extensively in his “Tales of Known Space” setting, where they are a way to propel a vessel to speeds where ram scoops can be used. These light sails are almost always pushed by giant lasers, placed either on tracks on Mercury, or on asteroids in the belt between Mars and Earth. In one story, “The Mote in God’s Eye”, written with Jerry Pournelle and set in Pournelle's own universe, the first encounter with an intelligent alien species is in the form of a meeting with a laser-pushed light sail.

There are several types of light sails – in the basic model the craft uses only light from stars to generate push. This is the model used by Cordwainer Smith. Acceleration is only provided as long as the light is intense, that is in the inner parts of solar systems. In those areas, light sails may be a cheap and efficient way of getting about. Acceleration is low, and the maximum speed is usually guessed to be about 25% of the speed of light. As the distance to the photon source increases, so does the efficiency of the light sail needed to sustain significant acceleration. There might very well be a point where a sail’s degree of push is countered equally by the friction of the incoming interstellar matter in the craft’s path.

The simplest form of light sail is a big circle or square with the craft dragged along on wires. This model is used in Robert Forward’s “Rocheworld”, where buckling of the sail occurs, but with no great concern from the crew. The danger of having the sail flap around or collapse in the fluctuating photon stream from the Sun seems to be a problem considered only in the real world, where ways of reinforcing the sail with poles seem popular.

Other forms of sail include the laser-powered light sail, where a giant laser or system of lasers bombards the sail with monochromatic light. This is favourable since it is easier to create efficient reflectors of monochromatic light compared to reflectors of ordinary sunlight composed of a spectrum of wavelengths. To be effective, the laser will have to be enormous and have its beam focused before the light reaches the sail. The laser-powered solar sail is seemingly the only way of getting a light sail to be effective on interstellar journeys. Larry Niven's early “Tales of Known Space” have giant laser batteries on Mercury, surrounded by a loose net of solar collectors.

One proposal by Robert Forward involves a lens the size of Texas (a Fresnel zone) placed between Saturn and Neptune, which focuses the lights from several thousand solar collectors in orbit round Mercury. These solar-pumped lasers will have a collective power of 65 GW. The light sail itself is a two-stage one, where the large outer sail is slightly bowl-shaped. It will be separated from (but very likely still connected to) the inner sail, upon arrival at Alpha Centauri (or Barnard’s Star?). The large outer sail then focuses the laser beam back onto the inner sail, thus braking the craft. The downside to laser-powered light sails is the tremendous laser power needed to propel the craft over interstellar distances.

Alternatives to light sails have been proposed. The microwave sail is also an idea from Robert Forward. A tiny spacecraft, the Starwisp, is propelled by microwaves transmitted from a solar-powered satellite in Earth orbit. The craft would be mostly a 1 km diameter mesh sail covered with microcircuitry, weighing only a few grams. The beam power should be 65–100 GW, pushing the 4–5 g craft to 20% of the speed of light, using a Fresnel-zone type lens to focus the microwave beam.

Particle sails are not exactly sail craft, but still use the concept of catching a beam from a ground-based emitter to gain acceleration. The beam in this case is a stream of heavier, slower particles like protons, emitted from a fusion reactor as plasma. The beam would disperse quickly, but the push gained could be up to 1000 g propelling the craft to 1/3 of the speed of light before the effect dissipates. The beam-projection limitations would make interstellar missions a one-way venture.

The magnetic sail is a use of Lenz’s Law (flux will attempt to remain constant in a electrified wire loop). The loop should be made from superconducting wires, and will expand to a circle when powered. The craft will be attached to this loop. Charged particles meeting the loop or magsail at other angles than parallel to the magnetic field will transfer some of their momentum to the field and thus push the craft. A magsail weighing 36 tons could receive accelerations of 0.0001 m/s to 0.009 m/s varying with the orientation of the sails. The mag loop is very small compared to a standard light sail, being only about 10 km in diameter.

Solar sails are very close to being non-fiction. The Russians have conducted the Znamya tests of light thin-film applications in space. The tests seemed to be for Earth-surface illuminatory purposes, but were also a test of alternative propulsion methods. In the United States, a light, bowl-shaped object was lifted 20 m using a laser beam. In Europe, ESA and DLR have developed solar-sail technology small enough to be carried into space and light enough to enable efficient sailing. A 20 m x 20 m model consisting of aluminum-coated sail segments with carbon-fibre-reinforced plastic hooks has been manufactured and tested.

LOS ANGELES — Four years after its first solar sail ended up in the ocean instead of orbit, The Planetary Society announced Monday that by the end of 2010 it will try again to launch a spacecraft that will be propelled by the subtle pressure of sunlight.

LightSail-1 is envisioned as the first of a trio of solar sail craft in a project boosted by an anonymous $1 million donation, according to the space advocacy organization co-founded by the late astronomer Carl Sagan.

Solar sails are theorized as a way to achieve long-duration interstellar flight.

A solar sail would be propelled by the pressure of light – photons pushing against a surface – not the stream of ionized gas known as the solar wind.

The spacecraft would accelerate very slowly but eventually reach tremendous speeds because of the constant pressure.

"It's a low acceleration but it's continuous, whereas a rocket is a high acceleration but it ends quickly," Louis D. Friedman, the society's executive director, said in a telephone interview from Washington, D.C.

"Eventually you'll have these missions lasting many years, reaching speeds approaching 100,000 mph, getting out of the solar system in five years instead of 25 years," he said.

The Planetary Society's first solar sail, Cosmos 1, never reached orbit. It was launched in 2005 aboard a converted missile fired from a Russian submarine beneath the Barents Sea. The launch vehicle failed and fell into the ocean.

The new project will be based on a micro-satellite technology known as CubeSat that was developed to give universities access to space.

LightSail-1 will be built around three CubeSat modules. Together, the modules will be "bigger than a toaster, smaller than a breadbox," Friedman said.

The spacecraft will launch with its four triangular Mylar sails packaged in a volume equal to about three quarts and unfurl to an area of more than 340 square feet, resembling a giant diamond-shaped kite.

It will orbit at an altitude of nearly 500 miles and operate for just a few days to determine if it can be controlled and to measure the orbital acceleration, Friedman said.

The mission cost will be about $1.8 million and all funds will come from private sources, he said.

LightSail-1 will be built by Stellar Exploration Inc. of San Luis Obispo, Calif.

Friedman said the launch vehicle has not been chosen yet. Because of its small size, the spacecraft can "piggyback" on another mission's rocket as long as it is going to the right orbit, Friedman said.

The LightSail-2 spacecraft is planned for higher altitude orbits, and LightSail-3 is intended to be sent farther into space. Both missions will be planned to last much longer than the first.

The society was not deterred by its previous failure and has not bitten off more than it can chew, Friedman said.

"There's always a little bit of arrogance in anybody building a spacecraft," he said. "These things are counterintuitive in so many ways, but you believe you can do it and when we succeed it's incredibly rewarding."

The society was founded in 1980 by Sagan, former Jet Propulsion Laboratory director Bruce Murray, and Friedman, a veteran of deep space mission planning at JPL.

* Preliminary experiments have been performed on two particularly interesting concepts in spaceflight propulsion. "Lightsails" involve traveling between planets using a large lightweight reflective "sail" that captures light from the Sun or a large laser. "Lightcraft" involve blasting payloads into orbit on a column of laser or microwave energy. This chapter describes the lightsail and lightcraft concepts, and outlines progress in these fields.

LIGHTSAIL CONCEPTS * Light is emitted in the form of particles called photons. Photons have momentum, and when they are reflected off a surface, they transfer momentum to that surface. In principle, if a spacecraft is tethered to a large reflecting surface, or "lightsail", sunlight falling on that surface would provide a gentle but continuous pressure that could propel the spacecraft between planets without any need to carry propellants for primary propulsion.

The concept of lightsail spacecraft has been around for decades. The first formal description of the idea was published in 1924 by the Russian spaceflight pioneer Konstantin Tsiokolvsky and his colleague, Friedrich Tsander. They wrote of "using tremendous mirrors of very thin sheets", and Tsander suggested that the sheets could be made of thin plastic covered by a metal thin film.

The basic principles that have guided design of lightsail spacecraft were first outlined by an engineer named Carl Willey. He published his ideas in an article written under the pseudonym "Russell Sanders" and published in ASTOUNDING SCIENCE FICTION magazine in May 1951. The idea attracted little attention until 1963, when British science fiction writer Arthur C. Clarke published a story titled THE WIND FROM THE SUN, which envisioned a lightsail race in space. The story was illustrated by the prominent space artist Bob McCall, and helped add lightsails to the long list of new space technologies investigated in the spacehappy enthusiasm of the early 1960s. Within a few years, some lightsail enthusiasts suggested using lasers or possibly microwave beamed power stations to give a lightsail a boost.

In 1976, NASA JPL conducted studies on a lightsail space probe to fly by Halley's Comet during its return to the inner solar system. They considered an 850 kilogram spacecraft, propelled by a square sail 800 meters on a side. The concept was too ambitious and the studies were quickly shelved.

* NASA's disillusionment with lightsails was not unreasonable, since there are serious practical obstacles to actually building one that works. The problem is that the pressure of sunlight is very small, which means that a very large reflecting surface of very little mass is required. This has a number of troublesome implications:

The reflecting material has to be very thin and light, usually being described as several times thinner than kitchen plastic wrap -- but still highly reflective on one side and very black and radiative on the other. It must also resist damage and, if damaged, resist tearing. Such a lightweight sail cannot be deployed in low Earth orbit, since trace atmospheric gases would degrade or destroy it. The sail must be able to survive trajectories that bring it in towards the Sun, where it will get the most propulsive kick for a journey into deep space, but where it will also be subjected to intense heat and the flow of charged "solar wind" particles from the Sun's corona.

Finding an adequate sail material has been difficult. The optimum solution would be a film that consists of a single, huge interlinked and interwoven molecule, but such an advanced material is a fantasy at present.

The sail has to be moveable to allow the spacecraft to maneuver. For example, if the sail is facing the Sun, sunlight pressure propels it in a line straight away from the Sun. If the sail is oriented at an angle of 45 degrees to the Sun, the solar photons bounce off at a right angle, and the momentum transferred sends the sail into space at a 45 degree angle from the line straight to the Sun.

The sail and support assembly have to be neatly packaged for launch, and then deployed in space in high orbit without wadding up or tearing the sail material. The material has to be stowed in such a way that avoids hard creases that would weaken the sail material.

Interest in lightsails began to revive in the late 1980s. There have been a number of interesting designs that attempt to address the control, packaging, and deployment issues, including the "kite", "fan", "sunflower", and "heligyro" lightsail configurations.

In the kite configuration, the sail is strung along spars in the form of a cross, with the spacecraft module in the center. The kite is launched with one spar fixed and the sail material folded to it. Once in deployment orbit, the other spar is extended, segment by segment, to unfold the sail. Small triangular fins are deployed at the corners of the kite to provide attitude control.

In the fan configuration, a disk-shaped sail is supported by radial spokes, with the spacecraft module at the hub. At launch, the sail material and the flexible spokes are coiled around the hub, and then unfurl when in deployment orbit. The spacecraft module controls the sail with motors that adjust the position of the spokes.

In the sunflower configuration, the sail is supported by a hoop around the edge, with the spacecraft module connected to the sail by a tether at the center and linked by guy wires to the hoop. The hoop is made of sections that are connected end to end, and folded in a zigzag fashion before launch. Once in deployment orbit, the hoop unfolds and spreads out the sail. The spacecraft module uses the guy wires to control the orientation of the hoop.

The heligyro configuration is unusual in that it requires no structural support. In the heligyro, the sail is in the form of multiple long, narrow blades that are simply unrolled away from the central spacecraft module. The entire spacecraft spins to keep the blades straight. The spacecraft module can pivot the blades to provide attitude control, though the heligyro's maneuverability is relatively limited. The configuration is most appropriate for deep-space flyby missions, where maneuverability is not a high priority.

* Practical experience with deploying such large structures in space is very limited. In 1993, a Russian Progress supply spacecraft used an electric motor to deploy a flexible mirror 20 meters in diameter. The mirror was intended to test the design of large mirrors that could be put into orbit to provide more sunlight for Russia's northern regions in the winter. A second mirror test using another Progress failed when the mirror couldn't be deployed.

Recent experiments in inflatable spacecraft structures have obvious applications in lightsail spacecraft technology. In May 1996, the NASA space shuttle deployed an experimental inflatable radio antenna. The experiment was not entirely successful, and work on space inflatables and other deployable structures continues.

LIGHTSAIL MISSIONS* In the early 1990s, a private group tried to promote a lightsail race in which commercial sponsors would send lightsail spacecraft to Mars. Nobody bit, but a privately-funded Russian-American effort later attempted to perform the first space tests of lightsail technology.

The "Cosmos 1" program was directed by the US Planetary Society, which has a long tradition of contacts with Russian space organizations. The Russian Babakin Space & Research Center built the experimental lightsail spacecraft, and also handled the launches. Electronics systems were implemented by the Russian Space Research Organization. Funding, on the order of several million USD, was formally provided to the Planetary Society by Cosmos Studios INC, which was set up by Ann Druyun, widow of Dr. Carl Sagan, the society's co-founder. The money was actually provided by Joseph P. Firmage, an Internet entrepreneur who founded the USWeb company.

The 40 kilogram Cosmos 1 spacecraft was a hybrid design, with features of both the sunflower and the heligyro configuration. It consisted of eight triangular panels connected to a central hub, with each panel supported by inflatable tubing around its edge. Each petal was about 15 meters long, giving the deployed satellite a diameter of 30 meters, and the sail material consisted of aluminized mylar 5 microns (millionths of a meter) thick. A cold gas thruster system was used to "spin up" the spacecraft to 95 RPM to aid deployment. Each petal could be rotated from the hub to change the direction of thrust.

* The initial plan was to launch a suborbital deployment test, followed by a full orbital mission. The deployment test was to feature a spacecraft with only two petals and carrying a camera to observe deployment. Telemetry was not implemented, with the camera parachuted back to Earth on a re-entry vehicle.

The full flight test envisioned launch of the Cosmos 1 spacecraft into a circular orbit with an altitude of 850 kilometers at an inclination of 78 degrees. The satellite was expected to obtain an acceleration of 10 meters a day under optimum conditions, and was fitted with a sensitive accelerometer to observe the actual levels of acceleration. After a few weeks of learning how to control the spacecraft, lightsail pressure was to be used to raise its orbit.

Another element of the test flight was to beam microwave power on it from the NASA 70-meter-diameter Deep Space Network antenna at Goldstone, California, to determine how beamed power might be used to boost the lightsail. Incidentally, ground tests of using microwave power to drive lightsail material demonstrated substantially more thrust than expected, with the reason finally determined to be that the beamed power was causing the lightsail to outgas materials. Lightsail researchers have expressed interest in the idea of actually building lightsails that take advantage of this "rocket" effect for initial boost.

The flights were expected to last at least three months. The missions were to be launched by a Russian "Volna" booster, a modified "SS-N-18" submarine-launched ballistic missile, shot from a Russian "Kalmar / Delta III" class submarine from the Barents Sea.

The first suborbital test shot was on 20 July 2001. Unfortunately, due to the failure of the booster's third stage, the Cosmos 1 spacecraft was never deployed, dooming the mission. This was disappointing, but valuable experience had been gained in setting up the mission and there had been no problem with the spacecraft as such. The second shot, on 21 June 2005, was to put a sail into full orbit, but once again there was a booster failure and the payload fell back to Earth. Status of the program is now unclear.

* Despite the problems, the Cosmos 1 tests reflect increasing interest in lightsail propulsion. The interest is based on new technologies and concepts. New sail materials, such as metallized sheets of carbon fiber mesh, have been proposed. A very important factor in the development of lightsails was the rise of interest in the 1990s for very small and cheap "microspacecraft" and "nanospacecraft".

With modern advances in microelectronics and micromachining, small spacecraft can perform useful missions. The low cost of such small spacecraft makes funding less of a problem, and their small size means that small lightsails can do a useful job. Lightsails seem particularly useful for small spacecraft, since in principle antennas, sensor arrays, and other useful subsystems could be integrated into the sail itself, reducing the overall weight and complexity of the spacecraft.

Along with new technologies are new concepts for using solar sails. While they were originally conceived as a means of deep-space propulsion, they are now increasingly seen as an attractive technology for other purposes, such as orbital station-keeping. The US National Oceanic & Atmospheric Administration (NOAA), with financial support from the USAF, has considered an operational lightsail spacecraft, named "Geostorms", to monitor solar storms, which can disrupt communications and power grids on Earth, and provide advance warning when one occurs.

The current concept envisions launching the spacecraft into an orbit around the Sun, 1.5 million kilometers closer to the Sun than the Earth. On arrival into this orbit, Geostorms will then unfurl a sail that will allow it to drop its orbit 1.5 million kilometers closer to the Sun, increasing the warning time for solar storms by almost an hour. Similar station-keeping lightsails could be used to maintain satellites in orbits at high latitudes over the Earth.

ADVANCED LIGHTSAIL TECHNOLOGIES / MAGSAILS* Station-keeping missions such as Geostorms do not require ultralight sail materials, and current materials, such as Kapton, can do the job. Unfortunately, even though Kapton has a mass of only about 12 grams per square meter, that's still too heavy for most other missions.

There's obvious room for improvement in sail materials. While Kapton can't be produced in sheets much thinner than 8 microns, Mylar polymer can be produced in mass quantity in sheets only half a micron thick. Mylar is not the best material for building a lightsail, since solar ultraviolet radiation breaks it down, but it does suggest that lightweight materials appropriate for lightsails can and will be synthesized.

A mission to Mercury could be conducted by a lightsail spacecraft with a sail 100 meters on a side and a density of less than 10 grams per square meter. However, really making use of lightsail technology requires a material with a density of less than 5 grams per square meter, and lightsail enthusiasts regard a density of 1.5 grams per square meter as the "holy grail" of their work. NASA JPL has conducted modest research on lightsails, with much of the effort focused on improved materials.

With such advanced sail materials, lightsail spacecraft with sails measuring about 200 meters on a side could be placed in orbits to observe the poles of the Sun, and perform flyby missions of the outer planets. For outer planet missions, a lightsail spacecraft would loop in towards the Sun, approaching about as close as the orbit of Mercury, and use its sail to obtain a strong boost from the intense solar radiation. The spacecraft would then arc out into deep space, discarding the sail once it reached the orbit of Jupiter, since sunlight pressure is too weak beyond that boundary to be useful.

NASA has organized discussions of a lightsail mission that would fly beyond the planets. The Interstellar Probe would be sent outside "heliopause", the boundary of our solar system, to a distance of 200 astronomical units (AU, where one AU is the distance from Earth to Sun), where it would perform studies of the interstellar medium. The discussions have also considered a more ambitious Interstellar Probe mission that would carry a telescope to a distance of 550 AU from the Sun, where it could use the Sun as a "gravitational lens" to perform high-resolution observations of distant cosmic objects.

The next step beyond sending a lightsail spacecraft into interstellar space is a journey all the way to a nearby star. A NASA investigation conducted in 1998 concluded that a lightsail spacecraft was the only technology that was close enough to practical development to be considered for such a mission. However, a lightsail star probe is still a significant technical challenge. The sail material would have to be extraordinarily light, the sail would have to be very large, and since the Sun's light becomes insignificant for propulsion beyond the orbit of Jupiter, the lightsail would have to be driven by a laser array of unprecedented size and power. This scenario is discussed in more detail in the next chapter.

* An interesting variation on the lightsail concept, known as the "magsail", was proposed by two aerospace engineers, Dana Andrews and the well-known Robert Zubrin. Instead of using a large reflective sail to catch solar photons, the magsail would use a loop of superconducting cable, as large as 30 kilometers in diameter, to generate a magnetic field that would catch solar wind particles. The advantage of the magsail is that it could obtain 50 times more propulsive effect than a comparable lightsail. The disadvantages are that it requires a huge superconducting cable, which is not practical at present, and an energy source to set up the magnetic field.

A team led by Robert Winglee of the University of Washington has proposed a different form of magsail, with the name "mini-magnetospheric plasma propulsion (M2P2)". Instead of a huge superconducting magnet, M2P2 involves a spacecraft with a large magnetic coil that generates a magnetic field around the spacecraft. Plasma is injected into the magnetic field, causing it to expand into a large tenuous magnetic field, possibly 40 kilometers across, that traps solar wind particles. In principle, M2P2 can be built with existing technology, though it does require power for the coil and a small quantity of gas as a source of plasma.

LASER LIGHTCRAFT* If lightsails are a subtle means of propulsion, lightcraft are very much the opposite, involving payloads launched into space on an intense beam of light.

The concept of the lightcraft owes much to one man, Leik Myrabo, an associate professor of mechanical engineering at the Rensselaer Polytechnic Institute (RPI) in Troy, New York. During the 1980s, Myrabo's interest in using a powerful laser to boost spacecraft into orbit progressed until by 1990 he and his colleagues at RPI were investigating concepts in detail, with backing from NASA and the Strategic Defense Initiative Organization (SDIO).

According to Myrabo, such a laser-boosted launch system offers much lower launch costs than a chemical rocket system. Since the spacecraft obtains most of its propulsive energy from external sources, a larger part of the spacecraft can be devoted to payload instead of fuel. Since the spacecraft doesn't incorporate a large high-thrust rocket motor, it also is potentially more reliable.

* Myrabo's initial concepts were based on lightcraft propelled by a large laser. Early experiments envisioned ground-based lasers, but long-range concepts considered the use of orbiting lasers, built as part of a solar power satellite (SPS) system.

The first designs for laser lightcraft defined a cone-shaped spacecraft, surrounded near its base by a ring shroud. Lightcraft designs propelled by a ground-based laser have a spindle-shaped baseplate, while those propelled by an orbiting laser have a flatter baseplate. In either case, the lightcraft carries a relatively small fuel load of hydrogen, as well as typical spacecraft systems such as control electronics, communications, attitude-control thrusters, and so on.

Lightcraft operating scenarios varied, but in a representative scenario using a ground-based laser, a lightcraft begins its flight by being catapulted into the air by a compressed-air charge. Laser light is shined onto its bottom, and the reflective ring shroud focuses the light to below the tip of the baseplate spindle. The focused energy is so intense that the hot air ionizes, creating a plasma arc that explodes outward and blasts the lightcraft upward at very high acceleration until the lightcraft reaches about Mach 1. At this speed, air is flowing over the conical front of the spacecraft, compressing the airflow and directing it into the ring shroud. The lightcraft then begins operating as a type of ramjet engine, with the conical spacecraft front acting as the ramjet centerbody, and the ring shroud acting as the ramjet duct.

For ramjet operation, the configuration of the shroud is adjusted to focus the laser light onto "igniters" underneath the ring that energize the air flowing into the shroud into plasma, providing thrust for the lightcraft. As the lightcraft accelerates to Mach 11, the ramjet operates in various "pulsed detonation" modes, in which thrust is obtained through a train of plasma explosions. During this flight phase, the airflow through the ramjet duct shifts from subsonic to supersonic, with the lightcraft then operating as a "supersonic combustion ramjet (scramjet)".

Above Mach 11, the lightcraft uses magnetic fields to compress the intake air, and injects hydrogen into the plasma flow to maintain thrust. Power for magnetic field generation is obtained directly from the intense plasma flow over the lightcraft through "magneto-hydrodynamic (MHD)" principles. MHD electrical power generation is conceptually simple, though difficult to achieve in practice because of the extreme nature of hot plasmas. A plasma is an ionized gas, consisting of free electrons and positive ions. If this plasma flows through a magnetic field, the electrons and positive ions are driven apart, creating an electrical potential difference. If an electrical circuit connection is placed at the edges of the plasma stream, electrons flow as a current through the circuit's load and then recombine with the positive ions on the other side of the circuit. The lightcraft MHD system generates large amounts of electric power, enough to produce strong magnetic fields to control the raging plasma flow.

By the time the lightcraft reaches an altitude where there is not enough atmosphere to sustain an air-breathing engine, it has a speed of Mach 25. It then uses a laser-powered rocket engine, with hydrogen exhaust heated by the laser beam back on the ground, for final orbital injection.

Scenarios envisioning the use of an orbiting laser instead of a ground-based laser were similar, except that of course the light was shined down on top of the lightcraft. The design was modified accordingly to allow reflection of the beam through the ring duct to a center point below the baseplate. Lightcraft designs imply the development of extremely precise and high-quality mirrors. If more than the smallest fraction of the intense laser light were absorbed by the lightcraft rather than reflected, the lightcraft would be incinerated immediately.

* Myrabo and his colleagues understood that the lightcraft was a long-range concept, and focused their research on basic physics and technology, hopefully leading up to small-scale demonstrations. They managed to show that a lightcraft system could produce thrust, but the US Strategic Defense Organization abandoned work on a 100 megawatt (MW) laser system that was needed for practical operation.

That left the laser option a dead end for the short run. Myrabo was undiscouraged, however, and joined with Yuri Raizer of the Russian Academy of Sciences to pursue another option: microwave power.

MICROWAVE LIGHTCRAFT / LASER LIGHTCRAFT TESTS * High power microwave (HPM) transmitter arrays are much more practical than high power lasers at present, and so HPM seemed like a better option for the short term. Myrabo and Raizer obtained funding from the US Space Studies Institute (SSI) to conduct studies. Their concepts for HPM lightcraft had much in common with those devised for the laser lightcraft.

HPM lightcraft development concepts focused on a ground-based microwave transmitter array, instead of a more ambitious microwave satellite power system. Early concepts for HPM lightcraft were basically an evolution of the laser lightcraft concept. The major difference was that instead of using reflectors to manipulate laser power, the HPM lightcraft uses an antenna array to transform microwave energy into propulsive power.

In this concept, the bottom surface of the HPM lightcraft is embedded with a large number of vertically-oriented wires, each acting as a receiving dipole, and tuned to the HPM transmitter wavelength. This "super-igniter array" absorbs the microwave energy and generates enough heat to ionize the atmosphere underneath, creating a plasma blast to drive the spacecraft upward.

Much like the laser lightcraft, at higher speeds the microwave energy sustains pulsed operation through the HPM lightcraft's ring duct. At high altitudes, an auxiliary orbiting microwave beam satellite heats hydrogen to provide pure rocket propulsion. An alternate scenario envisioned a self-contained chemical rocket for propulsion out of the atmosphere.

* In the SSI studies, Myrabo envisioned launching a network of very small "microsats" using microwave energy. Large numbers of microsats would be put in orbit to construct orbiting communications, earth resources mapping, weather observation, or military reconnaissance networks.

HPM microsat concepts envisioned a spacecraft weighing only 15 kilograms unfueled, including a 3 kilogram payload. Hydrogen fuel capacity was 15 kilograms of hydrogen fuel at launch, for a total launch mass of 30 kilograms. The microsat was built with lightweight carbon composites, with a temperature-resistant silicon carbide overcoat to protect the spacecraft. Subsystems included a 75 watt solar panel, rechargeable battery, communications and guidance electronics, magneto-optic data storage, and attitude control and pointing system.

The ground-based HPM array required to launch this microsat was 550 meters in diameter and operated at 220 GHz, a band where the atmosphere is mostly transparent to microwave energy. Beam power was 30 MW, with a maximum range of 500 to 800 kilometers.

* HPM design concepts didn't stop there, however. Later work was more radical. Advanced HPM lightcraft were disk-shaped, resembling 1950s flying saucers, though in most concepts they flew top-first, not sideways like a frisbee.

Scenarios for such advanced HPM lightcraft involved energy provided by orbiting power satellites. The lightcraft converted the microwave energy into useful power with "rectifying antennas (rectennas)", and had a ring of superconducting magnets underneath its rim to generate intense magnetic fields to control air and plasma flow, eliminating the need for a ring duct. Electrodes were also arranged around the edge of the disk to obtain MHD power from plasma flow.

The lightcraft overcame the air resistance that would seem unavoidable in a top-first flight configuration by focusing some of the microwave energy above the spacecraft. The plasma generated acted as an "aerospike", creating a shockwave cone to divert airflow around the lightcraft. Myrabo and his group considered alternate scenarios for such HPM lightcraft, such as high-speed air transport.

* Despite all this effort, Myrabo's lightcraft remained mostly paper studies, with small-scale physics experiments conducted to demonstrate the feasibility of various features of the scheme.

However, in 1996, Myrabo once again obtained US military funding to actually fly small lightcraft models, working in conjunction with Franklin B. Mead of the US Air Force Research Laboratory (AFRL). The AFRL sponsored small-scale tests of lightcraft propulsion at White Sands Missile Range, using a US Army 10 kilowatt pulsed carbon-dioxide laser. The tests were performed on beautifully machined solid aluminum models, ranging from 10 to 15 centimeters across and weighing about 50 grams.

Each laser lightcraft model resembled an acorn with a cone grafted onto its cap. A model was spun up with a nitrogen gas jet before launch to keep it stable in flight, and then kilojoule laser bursts, pulsed at a rate of 28 times per second, were fired on its bottom shield. The shield focused the light to ionize the air at the bottom of the lightcraft into a plasma, blasting the model upward.

Initial flights were performed with a guide wire. The first free flight was performed in November 1997, and was followed by others that successively pushed the lightcraft higher and higher, though the limit was about 30 meters because of range safety problems. The research team was hoping to obtain a more powerful laser for more aggressive flight tests, but it appears the effort has gone quiet. Some conspiracy enthusiasts suspect that the US government may have developed microwave lightcraft, suggesting that sightings of such machines are linked to UFO incidents.

DYSON'S SPACE BUTTERFLY* Freeman Dyson has described an interesting extension of lightsail and lightcraft spacecraft, which he calls the "space butterfly".

It is likely that humans will be able eventually to manipulate the genetics of any organism, and even create new organisms where the distinction between organism and machine is blurred. While "living machines" are a fairly common gimmick in science-fiction stories, Dyson specifically envisions the development of a microspacecraft based on the butterfly.

His space butterfly would be born and raised on Earth as a caterpillar, and at the end of that phase of its life cycle would weave itself into a tough cocoon. The cocoon could then be blasted into space on a small laser. Once in orbit, the space butterfly would break out of the cocoon and spread its wings, which would operate as lightsails.

The space butterfly's eyes would be little telescopes, its antennas radio aerials, its long springy legs optimized for clinging to and wandering over small asteroids, and its mouth full of chemical sensors for sampling the minerals on such asteroids and the solar wind. It would have a brain optimized for its mission, and built-in sensors to allow it to navigate. Dyson believes such technology will be available in a few decades. While the timescale is arguable, it is certainly a very entertaining idea.

The electric solar wind sail, or electric sail for short, is a propulsion invention made in 2006 at the Kumpula Space Centre.

The electric sail is a new space propulsion concept which uses the solar wind momentum for producing thrust (Janhunen, P., Electric sail for spacecraft propulsion, AIAA Journal of Propulsion and Power, 20, 4, 763-764, 2004, Janhunen, P. and A. Sandroos, Simulation study of solar wind push on a charged wire: solar wind electric sail propulsion, Ann. Geophys., 25, 755-767, 2007). The electric sail is somewhat similar to the more well-known solar radiation pressure sail which is often called simply the solar sail.

A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Therefore the solar wind ions "see" the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or "flying" the resulting spacecraft electrically.

The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun. This is about 5000 times weaker than the solar radiation pressure. Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft's electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind. For example, by equipping a 1000 kg spacecraft with 100 such wires, one may produce acceleration of about 1 mm/s^2. After acting for one year, this acceleration would produce a significant final speed of 30 km/s. Smaller payloads could be moved quite fast in space using the electric sail, a Pluto flyby could occur in less than five years, for example. Alternatively, one might choose to move medium size payloads at ordinary 5-10 km/s speed, but with lowered propulsion costs because the mass that has to launched from Earth is small in the electric sail.

The main limitation of the electric sail is that since it uses the solar wind, it cannot produce much thrust inside a magnetosphere where there is no solar wind. Although the direction of the thrust is basically away from the Sun, the direction can be varied within some limits by inclining the sail. Tacking towards the Sun is therefore also possible.